It has been known to plate metal particles on a metal substrate by applying mechanical force sufficient to cause adhesion between the plating metal particles and the surface of the substrate. The mechanical force necessary to cause such adhesion is achieved by placing the plating metal particles, a solid impaction media (e.g. glass beads), materials which promote such plating, and a metal substrate in a rotating ball mill or a tumbling barrel. In this manner, the rotation of the ball mill or a tumbling barrel imparts kinetic energy to the impaction media which is transferred to the plating metal particles such that these particles are pounded into the surface of the substrate as a coating.
The early work in this field of mechanical plating was disclosed in U.S. Pat. Nos. 2,640,001, 2,640,002, Re 23,861, 2,689,808, and 2,723,204 all to Clayton et al. Typically, these mechanical plating processes were undertaken in the presence of a liquid which contains promoter chemicals such as unsaturated fatty acids, film-forming materials, and surfactants. U.S. Pat. No. 3,460,977 to Golben discloses other promoter chemicals for mechanical plating. U.S. Pat. No. 3,328,197 to Simon teaches utilizing promoter chemicals in the form of a solid cake or bar which contain a combination of mechanical plating promoter chemicals. As the mechanical plating cycle progresses, the bar or cake dissolves at a rate which provides optimal amounts of promoter chemical to the mechanical plating process.
U.S. Pat. No. 3,268,356 to Simon ('356 patent) discloses incrementally adding the promoter chemical and/or the plating metal particles to the plating barrel in successive additions to optimize the density and uniformity of the plating metal coating over the entire substrate surface.
To prevent corrosion of thin mechanical plating coatings, i.e. coating up to 25 microns (1.0 mils), it has been suggested that a "sandwich" coating (e.g. a coating of zinc on tin on zinc) be applied to a substrate, as disclosed in U. Meyer's "Mechanical Plating Die Entwicklung des Verfahrens", Galvanotechnik, Vol 73, No. 9 (1982).
U.S. Pat. No. 3,531,315 to Golben ("'315 patent") discloses performing a mechanical plating process in the presence of a strong acid. Prior to the '315 patent, agitation of plating metal, impaction media, and substrate generally was conducted in the presence of weak organic acids such as citric acid. This required that the contents of the plating barrel be rinsed free of any strong acids used to clean or copper the parts before starting the citric acid-based plating process. With the process of the '315 patent, it was possible to conduct the mechanical plating process without need for intermediate rising steps, rendering the process extremely economical.
Gradually, it became desirable to use thicker (e.g. from about 1.0 to 5.3 mils compared to mechanical plating coatings which are 0. 1 to 1.0 mils thick) and heavier (e.g. from about 0.7 to 2.5 ounces per square foot) mechanically-applied metallic coatings. Such methods of applying thicker, heavier coatings came to be known as mechanical galvanizing processes. During the development of such mechanical galvanizing processes, it was found that enhanced adhesion of mechanical galvanizing coatings could be achieved by building up thin layers of mechanically plated metal. As taught by the '356 patent, such layered coatings were achieved by the incremental addition of plating metal powder to the process. As a result, the commonly utilized citric acid-based chemistry, such as that described by the '356 patent, could be employed in mechanical galvanizing. The pH of about 3.0 to 3.5 with this chemistry is less aggressive upon the metal powder, and the promoter chemicals can be introduced in bar form (see e.g. U.S. Pat. No. 3,328,197) which slowly disintegrates during the process and gradually releases the chemicals as galvanizing progresses. However, the organic acids and their salts are expensive and tend to complex heavy metal ions which hampers effective effluent treatment.
It was also desired to optimize mechanical galvanizing in accordance with the teachings of the '315 patent to secure the same advantages achieved by mechanically plating in a strong acid (i.e. eliminating the need for intermediate rinsing). However, the chemistry utilized with the process of the '315 patent is not amenable to incremental additions of metal powder, because the 0.5 to 1.5 operating pH in this system is too aggressive on the metal powder. In addition, the typically-used promoter chemicals were introduced in powder form at the start of the galvanizing process with no intervening additions. Utilizing this promoter chemistry in conjunction with the incremental addition of plating metal powder would result in an improper chemical environment at later stages of the process, causing the uncontrolled deposit of metal coatings. Consequently, the conditions necessary to apply successive layers of well consolidated, adherent particles could not be uniformly maintained.
U.S. Pat. No. 4,389,431 to Erismann ("'431 patent) adapted the process of the '315 patent to the incremental metal powder additions of mechanical galvanizing. This was achieved with two chemical promoter systems. The first is a flash promoter which coats the substrate with a thin adherent flash coating of a metal more noble than the plating metal prior to adding the plating metal to the system. The second continuing promoter is then incrementally added with some or all of the incremental additions of a finely divided mechanical plating metal, the layers of which are built up to effect mechanical galvanizing.
Despite this improvement, there continue to be problems with mechanical galvanizing coatings which are not encountered with mechanical plating coatings. One such problem encountered with the thicker mechanical galvanizing coatings is chipping, flaking, and cracking which becomes more of a problem as the thickness of the coating increases. This is a particularly big problem with larger parts which impact against each other and against the galvanizing barrel. On smaller parts, such as nails, the whole coating can flake or chip off when bent in accordance with ASTM Test ASTM B571, Standard Methods of Testing for "Adhesion of Metallic Coatings".